A conventional nuclear reactor includes a pressure vessel containing a nuclear reactor core. The vessel contains reactor water which is heated and boiled by the reactor core in an exemplary boiling water reactor (BWR) for generating reactor steam having a pressure of about 70 kg/cm.sup.2 for example.
In order to control the reactivity within the core, a plurality of conventional control rods containing a nuclear poison are selectively inserted into or withdrawn from the reactor core by conventional control rod drives. A typical control rod is relatively long and straight in the vertical direction and is joined to an actuator rod extending from the control rod drive. The control rod drive is typically mounted below the pressure vessel with the actuator rod extending upwardly through the lower head thereof. The length of the actuator rod is about equal to the length of the control rod so that the control rod may be fully inserted upwardly into the reactor core and fully withdrawn therefrom. The length of the control rod drive is also about equal to the length of the actuator rod so that the rod may be extended therefrom for inserting the control rod into the reactor core, and withdrawn therein so that the control rod may be withdrawn from the reactor core.
Accordingly, this vertical arrangement of control rod drive, actuator rod, and control rod has a combined length of about three times the stroke or length of the control rod itself. In order to allow for removal of the control rod drives from below the reactor pressure vessel, another, or fourth stroke length, is typically provided below the control rod drives in an open space to provide access to the control rod drives. The resulting combined vertical length of the control rod, actuator rod, and control rod drive, as well as the additional stroke length for access therebelow, results in a relatively long assembly which in turn requires a relatively tall reactor pressure vessel as well as containment building in which the pressure vessel is disposed. An attendant increase in complexity and cost thereof results.
However, the vertical arrangement of the control rods and the bottom-mounted control rod drives typically requires that the control rods be pressure driven upwardly against the force of gravity for Scram insertion into the core.